Have you checked this multiple times, are the results same all the time?

There would be transactions/ changes that were in progress when you took the backup and needs to be backed up.

What happens when you take another FULL backup and a Tlog backup following that?

To dig further, you may want to use "RESTORE HEADER ONLY" for the backup taken and review the DatabaseBackupLSN and CHECKPOINTLSN. This might give you a hint for why the LOG Backup is big.

Can also get the similar details from: "msdb..backupset"

Reviewing the above for both backup sets can provide more clarity.